NEW YORK (CNN/Money) - Besides slowing down your computer and subjecting you to annoying pop-ups, worm and virus attacks can do serious harm.

A key-stroke logger, for example, can get into your computer through security holes, and once aboard it can monitor keystrokes to look for patterns. When it sees one that indicates your PC is in communication with your bank, it potentially could find your account number and password.

It might be weeks later -- when you get an unexpected bill or examine your monthly bank statement -- that you learn your identity has been stolen.

That's just one type of attack in a virtual world-wide cyber war. Legions of hackers relentlessly bombard networks for fun and profit with viruses, worms, and spyware, probing for weaknesses, turning every computer into a battleground.

Users can call on weapons of their own for protection. Here are six steps to take that will help keep your system from becoming a casualty.

1) Install a firewall: The first line of defense, a firewall, is a program that:

• Keeps the bad guys out. A state-of-the-art firewall "stealths your computer" as Fred Felman, VP of marketing for Zone Labs, which sells security software, puts it. "It hides your machine from hackers," so viruses can't enter.
• Keeps the good stuff in. A firewall prevents passwords, account numbers, and other sensitive information from being transmitted without your permission. Users themselves control when to transfer this kind of data. A good firewall with outbound filtering doesn't allow Trojan Horse programs or spyware to "call home."

Some firewalls, ones at Zone Labs and Sygate, for example, are available free of charge and take mere minutes to install. Just go to their Web site, click on the Zone Alarm link, hit the "free download" button, and follow some easy directions.

Other firewalls sell for about $40. James Manning, director of operations for Aluria Software, whose main product is an antispyware program, says the free firewalls help a little, but the pay firewalls support many more features, like e-mail attachment protection, advanced IP blocking, ad blocking, popup-window protection, and provide more automatic functions.

2) Use antivirus software: Viruses, unauthorized computer codes attached to a program or portions of a computer system that reproduce and spread from one computer to another, can destroy information stored on the computer, and interrupt operations. No matter how vigilant you are, sooner or later a virus will worm its way into your PC. An antivirus program detects and destroys this rogue code.

Software makers often package antivirus programs with their firewall. It's a natural complement. Firewalls help prevent viruses from entering your system; an antivirus program searches for and destroys any that get through.

Firewalls are an especially useful defense against brand new worms that antivirus programs do not yet recognize. After the virus's code becomes known, the antivirus software can deal with it. The reason why having a firewall matters is that there are so many new worms and viruses and variants.

The Norton AntiVirus 2004 software can be downloaded from the Symantec Web pages. It costs $49.95.

Both Symantec and Zone Labs also have complete security suites available that contain a firewall and anti-virus plus anti-spam software for about $60.

3) Use antispyware software: Spyware is any software program that aids in gathering information about a person or organization without their knowledge, and that can relay this information back to an unauthorized third party.

For the most part, users themselves open up the door and invite spyware in by downloading free software indiscriminately or by clicking on pop-ups or dialogue boxes.

Some spyware will take over your browser so that every time you launch it will go, not to your normal home page, but to another. Other spyware will merely help generate a spate of pop-up ads that can make Web surfing a chore. But it's the key-stroke logger spyware that can cause the most financial damage.

A couple of dozen anti-spyware downloads from various makers are available at Spychecker. Some are free, but most cost about $25.

4) Examine browser security settings: Make sure your security settings on Internet Explorer (the browser of choice for most users) are set adequately. Browser-based attacks occur when a user goes to a Web page that contains hidden code intended to sabotage a computer or compromise privacy.

A common method of attack involves phishing with e-mails that link to a malicious Web server. Many firewalls don't help because the attacks happen after the user clicks on the link. Traditional firewalls examine traffic coming into the network, but guarding against browser attacks requires that traffic leaving the network also be inspected.

To access security settings you click on Tools on the menu bar, then Internet Options on the pull-down menu. Then click on Security. Internet Explorer categorizes sites as Internet, Local Intranet, Trusted Site, or Restricted Site zones. You can adjust settings for each or use the Default Level button for pre-defined security settings from Microsoft.

The default setting of your browser, according to Manning, is usually medium-high, fine for most users. "If it's set too low, a lot will get in," he says.

5) Take advantage of security updates: Your Internet service provider and your browser periodically issue security updates. Often these are to patch holes that viruses can get through. Make sure you have all the latest fixes available.

Reputable software vendors dedicate parts of their Web sites to patches; if you don't have or use auto-update mechanisms in your software you can go there.

Check for updates frequently at http://www.microsoft.com/technet/security/default.mspx. You can also register for free e-mail notification for security update information at http://www.microsoft.com/technet/security/bulletin/notify.mspx.

6) Read your user licensing agreements: Every time you download a file from the net, you put your system at risk; this is how a lot of spyware enters. "You inadvertently agree to accept the spyware along with the program you're downloading," says Manning.

The agreement includes any other applications that come along with the desired software. Only if you recognize the additional programs and know they are safe should you complete the download.

"You wouldn't invite a stranger into your home," says Felman. "It's the same idea with your computer. Make sure the source is reliable by dealing with vendors you know or who are recommended by others you trust."

Computer protection is everyone's responsibility. Since we're all part of a vast interconnected system, says Felman, "We all have to be part of the solution."